This invention relates to a process for producing unsaturated aldehydes and unsaturated carboxylic acids. More particularly, the invention relates to a process for vapor-phase catalytic oxidation of at least one starting compound selected from propylene, isobutylene, t-butanol and methyl-t-butyl ether (hereinafter these compounds may be referred to as xe2x80x9cpropylene and the likexe2x80x9d) to produce corresponding unsaturated aldehydes and unsaturated carboxylic acids. When propylene is used as the starting compound, acrolein and acrylic acid are produced, and when isobutylene, t-butanol or methyl-t-butyl ether is used as the starting compound, methacrolein and methacrylic acid are formed.
Many improved catalysts for producing unsaturated aldehydes and unsaturated carboxylic acids with high efficiency by vapor-phase catalytic oxidation reaction of propylene and the like were heretofore proposed. For example, Sho 50(1975)-13308A-JP (=GB 1,444,659A) and Sho 50(1975)-47915A-JP (=GB 1,444,659A) disclosed catalysts whose essential components were Mo, Bi, Fe, Sb, Ni, and at least one another element selected from K, Rb and Cs. Sho 64(1989)-56634A-JP (=EP 304,867A) taught catalysts whose essential components were Mo, Bi, Fe and at least an element selected from Ni and Co.; Sho 56(1981)-52013B-JP (=GB 1,330,074A), the catalysts essentially containing Mo, Bi and Fe and at least an additional element selected from Mg, Ca, Zn, Cd and Ba; and Sho 56(1981)-23969B-JP (=GB 1,390,271), catalysts essentially containing Mo, Bi and Fe, and at least one element selected from Group IIA compounds and Group IIB compounds of the periodic table.
Industrial scale production of unsaturated aldehydes and unsaturated carboxylic acids by vapor-phase catalytic oxidation reaction of propylene and the like is subject to a number of problems, one of which is occurrence of local abnormally high temperature spots (hot spots) in the catalyst layers. Because the vapor-phase catalytic reaction of propylene and the like is extremely exothermic, hot spots may occur in the catalyst layers to induce over-oxidation or the excessive heat generation at the hot spots may cause deterioration of the catalyst. In the worst case, a run-away reaction may be induced. In particular, where concentration of starting material or space velocity is increased to raise productivity of the object products, the excessive heat generation takes place to make stable production of the object products difficult.
Various methods have been proposed to control occurrence of such hot spots or the excessive heat generation at the hot spots. For example, methods in which the catalyst at the hot spots was diluted with inert substances [Sho 43(1968)-24403B-JP, Sho 53(1978)-30688B-JP (=U.S. Pat. No. 3,801,634) and Sho 51(1976)-127013A-JP (=GB 1,529,384A)]; methods in which ring-formed catalysts were used [Sho 62(1987)-36739B-JP (=U.S. Pat. No. 4,438,217) and Sho 62-36740B-JP (=U.S. Pat. No. 4,511,671)]; a method in which two or more reaction zones were provided in each reaction tube [Sho 51(1976)-127013A-JP (=GB 1,529,384A)]; a method in which plural catalysts, which were prepared to have different activity levels by varying the amount and/or kind of alkali metals therein, were filled in the reaction tubes in such a manner that the activity level rose from the reactant gas-inlet side toward the outlet side [Sho 63(1988)-38331B-JP (=U.S. Pat. No. 4,837,360)] were included among the proposals.
However, those known methods have not completely solved the problems pertaining to the hot spots.
Accordingly, therefore, the object of the present invention is to provide a process for effectively inhibiting or controlling occurrence of hot spots or excessive heat generation at the hot spots to enable production of unsaturated aldehydes and unsaturated carboxylic acids at high yields.
We have discovered, after extensive studies, that when plural shaped catalysts exhibiting different activity levels are prepared by varying: [I] content of inert component of the shaped catalysts; and [II] at least one of the factors (a) occupation volume of the shaped catalysts, (b) kind and/or amount of the alkali metal(s) in the shaped catalysts and (c) calcining temperature of the shaped catalysts; and the catalysts are filled in each of reaction tubes in such a manner that the catalytic activity level rises from the reactant gas inlet side toward the outlet side, occurrence of hot spots or excessive heat generation at the hot spots can be effectively controlled, and in consequence unsaturated aldehydes and unsaturated carboxylic acids can be produced at high yields.
Thus, according to the invention, a process for preparing unsaturated aldehydes and unsaturated carboxylic acids through vapor-phase catalytic oxidation of at least one starting compound selected from propylene, isobutylene, t-butanol and methyl-t-butyl ether with molecular oxygen or a molecular oxygen-containing gas, using a fixed bed shell-and-tube type reactor filled with shaped catalysts is provided, which is characterized in that plural shaped catalysts exhibiting different activity levels are prepared by varying at least one of the factors: [I] content of inert component of the shaped catalyst and [II] (a) occupation volume of the shaped catalyst, (b) kind and/or amount of the alkali metal(s) in the shaped catalyst, and (c) calcining temperature of the shaped catalyst; and which catalysts are filled in each of the reaction tubes in such a manner that the catalytic activity level rises from the reactant gas-inlet side of each reaction tube toward the outlet side.
The term, activity level, as used herein is determined based on the conversion of the starting compound (propylene and the like).
The active components of the catalyst to be used in the present invention are not critical, so long as they are useful in vapor-phase catalytic oxidation reaction of propylene and the like, to produce the corresponding unsaturated aldehydes and unsaturated carboxylic acids. Whereas, complex oxides which are expressed by the following general formula (1) are particularly suitable:
MoaWbBicFedAeBfCgDhOx
(wherein Mo is molybdenum; W is tungsten; Bi is bismuth; Fe is iron; A is at least an element selected from cobalt and nickel; B is at least an element selected from phosphorus, antimony, boron, tin, cerium, niobium, lead, chromium and zinc; C is at least an element selected from alkali metal elements; D is at least an element selected from alkaline earth metal elements; and O is oxygen; a, b, c, d, e, f, g, h and x stand for atomic numbers of Mo, W, Bi, Fe, A, B, C, D and O, respectively; where a is 12, b is 0-5, c is 0.1-10, d is 0.1-10, e is 1-20, f is 0-5, g is 0.001-3, h is 0-5, and x is a numerical value determined by state of oxidation of each of the elements).
The complex oxides which are expressed by the general formula (1) are known, and can be prepared by heretofore known methods. More specifically, for example, Japanese Patent No. 2,659,839 (=U.S. Pat. No. 5,276,178) may be referred to.
The inert component to be used in the invention may be any that is inert to the reaction. For example, those generally used as inert carriers, such as silicon carbide, aluminum oxide (alumina), zirconium oxide (zorconia), titanium oxide (titania), silicon oxide-aluminium oxide (silica-alumina) and the like can be used. Of those, alumina and zirconia are conveniently used.
BET specific surface area of such inert substance desirably does not exceed 20 m2/g, preferably ranging 0.1-15 m2/g, inter alia, 0.5-10 m2/g. Where the specific surface area is large, CO and CO2 formation increases to decrease yield of the object products. Average particle diameter of the inert substance desirably does not exceed 200 xcexcm, preferably ranging 0.1-100 xcexcm, inter alia, 0.5-80 xcexcm. Large particle size is detrimental to its blendability with catalytically active components.
Shaped catalysts according to this invention can be prepared, for example, when a complex oxide expressed by the general formula (1) is used as the catalytically active component, by adding an inert substance to the complex oxide at an optional step during the preparation of a shaped catalyst comprising said complex oxide, and mixing it into the system. For example, a shaped solid catalyst can be prepared by a process comprising: adding starting materials containing the constituent elements as expressed in the general formula (1) to an aqueous medium and mixing; adding an inert substance to the resulting slurry; mixing; heating, drying; pulverizing; adding to the system an additive such as ammonium nitrate, cellulose, starch, polyvinyl alcohol, stearic acid and the like which are generally referred to as binders, together with water; kneading the system and thereafter shaping the kneaded mass into desired forms and calcining the same while passing air. In that occasion, as the first condition [I]. the amount of added inert substance is changed and, as the second condition [II], at least one of (a) the occupation volume of the shaped catalyst, (b) kind and/or amount of the alkali metal(s) in the shaped catalyst, and (c) calcining temperature of the shaped catalyst is changed, to provide plural shaped catalysts having different activity levels.
In an alternative method, shaped catalysts can be prepared by: adding starting materials containing the constituent elements as specified by the general formula (1) to an aqueous medium, mixing, heating the resulting slurry, drying, pulverizing, mixing the resulting powder with an inert substance, and then adding thereto an additive such an ammonium nitrate, cellulose, starch, polyvinyl alcohol, stearic acid and the like which are generally known as binders, together with water, kneading the system and shaping the kneaded mass into desired forms, followed by calcining while passing air. Plural catalysts of different activity levels can be prepared in that occasion, by changing the added amount of the inert substance as the first condition [I], and also by changing at least one of the second condition [II], (a) occupation volume of the shaped catalyst, (b) kind and/or amount of the alkali metal(s) in the shaped catalyst, and (c) calcining temperature of the shaped catalyst.
The content of inert substance is variable within the range of 0-80% by weight, preferably within the range of 0-70% by weight.
The term, xe2x80x9coccupation volume of the shaped catalystxe2x80x9d, as used herein means the apparent volume occupied by the shaped catalyst. Apparent volume signifies the volume including inside cavities of the shaped catalyst. Therefore, when shaped catalyst bodies are spherical or cylindrical, their occupation volumes equal the cubic volumes of the spheres or cylinders. Again, when the shaped catalyst are ring-formed, for example, their occupation volume equals the cubic volumes of the rings plus those of the spaces surrounded inside the rings. Thus, when (A) the shaped catalyst are spherical, their occupation volume is changeable by changing their diameters; (B) when the bodies are cylindrical, by changing their diameter and/or height; and (C) when they are ring-formed, by changing their outer diameter and/or height of the rings. When inside of each reaction tube is divided into plural reaction zones, it is convenient to fill any two adjacent reaction zones with shaped catalysts of differing activity levels, in such a manner that the ratio (V1/V2) between the occupation volume (V1) of a shaped catalyst filling the reaction zone closer to the reaction gas inlet side and the occupation volume (V2) of another shaped catalyst filling the reaction zone closer to the reaction gas outlet side falls within the range of 1.2/1-64/1, preferably 1.3/1-27/1.
The amount of alkali metal is variable within the range of, for example, the atomic ratio specified by the general formula (1). The calcining temperature also is variable within a range of 300-650xc2x0 C., preferably 400-600xc2x0 C. While the calcination time is not subject to any critical limitation, it normally ranges 1-24 hours, preferably 3-12 hours.
Form of the shaped catalyst according to the invention is subject to no critical limitation, which may be any of granules, pellets, spheres, cylinders, rings and the like.
Apparatus and conditions for practicing the vapor-phase catalytic oxidation reaction of the present invention are not critical. As the reactor, a generally used fixed bed reactor is used, and each of the reaction tubes in that reactor is divided into plural reaction zones, normally two or three, and the zones are filled with the shaped catalysts of different activity levels in such a manner that the activity level rises from the inlet side toward the outlet side of the reaction gas The reaction may be performed under those conditions that are generally selected for preparation of unsaturated aldehydes and unsaturated carboxylic acids through vapor-phase catalytic oxidation reaction. For example, the reaction is performed by contacting a gaseous mixture comprising 1-15 volume % of gaseous starting compound, i.e., propylene and the like (at least one compound selected from propylene, isobutylene, t-butanol and methyl-t-butyl ether), 1-10 volume % per volume % of the gaseous starting compound of molecu ar oxygen and the balance of an inert gas (e.g., nitrogen, carbon dioxide, steam or the like) serving as the diluent, with the shaped catalysts of the invention at temperatures ranging 250-450xc2x0 C. under pressures ranging 0.1-1 MPa and at space velocities ranging 300-5000 hxe2x88x921 (STP). While nitrogen, carbon dioxide, steam or the like can be used as the inert gas, use of steam is advantageous for improving yields of the object products, because steam has an effect of inhibiting formation of side products.
According to the process of this invention, occurrence of hot spots or excessive heat generation at the hot spots can be effectively inhibited, and in consequence the intended unsaturated aldehydes and unsaturated carboxylic acids can be obtained at high yield.